Thursday, October 16, 2008

Thoughts on laser cut dropouts..

Recently, while visiting the Brooklyn Machine Works shop, I had a look at a variety of Laser cut dropouts. I can't get into detail about the the designs I saw there, because I don't have pictures to illustrate them, and there was just TOO much awesomeness going on there to be described in one post.

Laser Cutting is the general technology used to churn out dropout after dropout for the majority of today's track and BMX frames. Obvious exceptions to this are the forged and machined examples found on some high end bicycles. For the most part however, the single thickness dropouts found on your track bike, were cut with a CNC driven laser.

Plasma cutting is another viable option, and may have been used in the past. However, the width of beam is slightly larger than with a laser, and therefore accuracy is less. With the ever decreasing price of laser cutting technology, its almost easy for designers to get carried away.

(BMW actually doesn't do any laser cutting in house. They do their designs in AutoCAD and contract another metal shop to do that)


I'm not going to name names, but here we have excessive laser cutting. Its as though the designer wanted some swoopy crescents to dress up his dropouts. A napkin was marked up with a fine tipped sharpie, and sent overseas. The factory reviewed said napkin, said "Sure thing chief!" and away they went with the CAM program. Never mind that the actual crescent shapes have no flow, and subsequently do nothing to create an air of classiness. Now, I don't personally know how much this will affect the strength of the design. I just get this vibe that its a bad idea.

A city ridden bicycle is subject to stresses many times greater than the riders weight when road imperfections are encountered. The axle pushes UP on the dropout with ridiculous amounts of force. These stresses occur at high frequency, every ride. These are the sort of forces that (over time) lead to dropout weldment failures, or similarly seat stay/seat tube junction failures.

A stress riser is essentially where localized forces can overcome the shear strength of a material. And nothing localizes forces more than sharp angles. The stress concentration factor goes up exponentially as the radius of a cut approaches zero. The idea of adding 8 tiny-radius-acute-angle cuts to a small piece of metal that is undergoing high frequency shear forces from the axle just seems like its ASKING to test the elastic modulus of the steel being used.

Just looking at that picture is bothering me enough that I have to model it. I MUST know just how much these cuts affect the strength of the part!

Here I'm starting by importing the photo of the unnamed bicycle dropout into a modeling program, rotating it and scaling it correctly. Then I just draw the existing dropout shape in a 2D sketch:


Now, lets suppress the photo and extrude the basic dropout. Shall we assume it's 5mm thick?


Looks decent.

Okay, here I'm just copying the random swoopy crescent shaped cuts in our unnamed dropout. Looking closely, there is only one straight line. The absolute randomness of these cuts really makes my brow furrow.


...and the final cutout:


Looks horrible doesn't it?
Anyways, on to the testing.

The simplest way to do this, will be to anchor the two faces where the dropout meets the rest of the frame. Next I'm going to put a very slight axle groove in the top of the dropout so that the forces of the axle can REALLY be localized. (as opposed to applying force along the entire upper edge of the dropout) Remember, I'm trying to simulate an axle impacting this thing.


Okay, lets assume our Proj-B test rider weighs about 190lbs. If he hits an object (like a wheel swallowing pothole in the South Bronx) then the force on the axle is going to be several times his mass. We don't actually need to use any particular value here, because I'm going to use the same value to test the dropout with and without the fancy cutouts, but lets assume that the force is twice the rider's weight, thats 1690N being applied at the upper edge of the dropout

This program has a handy stress analysis tool that allows one to calculate the Factor of Safety (FOS) of a part. Simply put the FOS is the Material's yield strength divided by the equivalent stress at a point.

-An FOS less than 1.0 at a location indicates that the material at that location has yielded.
-An FOS of 1.0 at a location indicates that the material at that location has just started to yield.
-An FOS larger than 1.0 at a location indicates that the material at that location has not yielded.

So, for the Plain old dropout, and a 1690N force, we get a FOS of: 5.25102
No problem. I would ride this all day.

The Von Mises stress distribution on the model look like this:


Lets repeat that test for the Fancy Cutout version:


Here our FOS is 4.43389 That's 15% less strength!

These pictures show the model in a total deformed state, though thats not actually the case for these applied forces. The dropout will NOT deform under these conditions. The program just uses this to illustrate how the stresses are distributed.

So what can we tell from this?

Well, its certainly more colourful. You can see where the stress is concentrated in this last picture. That lower crescent and the little arrow head appear to be areas of concern. It seems the cutout dropout is only 85% as strong as the solid one. Whether or not that is 'strong enough' is entirely subjective. Much like my opinion of the aesthetic design of this particular dropout.

I started playing around, finding the maximum force before the material would actually deform. It seems the solid dropout could withstand a force of 9kN or 2020lbs before it would yield.. and even then, just barely:


When that same force is applied to the cutout version, we see a couple more areas of deformation. Most notably is that little pointy arrowhead tip again:



I guess today I've learned that fancy cutouts aren't really as bad as I thought they were. I've also learned how easy it is for me to get sidetracked into theoretical testing and hypothesizing simply by staring at a picture of a bicycle for too long.

If any of this bicycle's designers are reading, please don't feel too slighted by my words. (I didn't even touch upon your PBR bike!) I am only doing this because I wish i was doing what you are doing.

8 comments:

mander said...

Respect

morgman said...

This is one of the best posts of all time.

Matthew said...

I was wondering about those Traitor frames... The paint jobs are interesting if not what everyone wants, but the overall construction leaves me wanting more.

Traitor Cycles said...

This is Kevin from Traitor Cycles. I am responding to this because a customer brought this up to me. I can totally respect the fact that you don't like the shape of the dropout and that is fine but you should know about our design process. We don't use napkins, we use solidworks which is similar to your program and we run the same modeling as you do so we are full aware of the stresses. Been doing this for over 8 years. I also own a mountain bike company called Transition Bikes (www.transitionbikes.com) and we design full suspension alloy bikes so we are full aware of stress points. I think you realized that the stress needed to break a dropout is not readily reproducible in the real world. If something that massive did happen you can be assured that your chainstays or seatstays would break before a dropout. Also a stress analysis like yours doesn't take into account wheels, tires, flex from the chainstays and the fact that you have two dropouts distributing the force as well as torsional forces. It is also intersting to note that on your test the highest stress area is on a rounded part of the dropout. There are tons of intricate cut dropouts with sharp angles that custom builders use without issue and the truth is that as long as there is enough material around it, it will never be an issue so you can have a little artistic freedom. I give you huge kudos for taking on a project like this and I have no problem with it, my only issue is with how you portrayed our R&D without knowing anything about us. In the end I am actually glad there are people like you out there as it helps keep people in check and can help educate the public. I have never seen anyone geek out on something like this to your level and it is pretty awesome. We just don't want to spread any wrong information about our company.

((lyledriver)) said...

Hi Kevin,

Thank you for taking the time to respond. It’s good to see company owners taking on this kind of role. I realize I was being unnecessarily inflammatory and nerdy the day I wrote that post. I was sitting around looking at bike after bike and I didn't really need to pick on yours as much as I did. The dropout design just really caught my eye, and I had to know how that design affected it. That was over a year ago, and I've since wised up a little after getting a couple of my own bike projects to prototype testing stage.

When I said "Its as though the designer wanted some swoopy crescents to dress up his dropouts. A napkin was marked up with a fine tipped sharpie, and sent overseas." I was using hyperbole. (Though I have heard of companies faxing paper sketches back and forth to manufacturers) I am stoked you guys use SolidWorks. You're right about the challenges of modeling real world stresses on bike stuff. I did an overly simplified test on my reverse engineered version of your dropout. And really, a theoretical loss of 15% dropout strength is nothing when you do take into consideration the fact that there are two of them bolted together into a solid unit with welds right beside them.

I didn't realize you were also involved with Transitions bikes. Now that I know that, I've got a few questions about the TrailorPark, TransAm and Bank dropouts... but maybe I should save that for email. Or we could discuss it over a post ride beer next time you're North of the line.

Again, thanks for taking my entirely subjective opinion with a grain of salt. Having said that, I think the Streetfighter dropout looks like a much cleaner design.

Traitor Cycles said...

No worries, and I think you are correct that some people do send designs over via napkin. I have seen some horrible stuff over there. Regarding the TOP and Bank stuff, just shoot me an email to my Transition account, it is my first name then our url for Transition Bikes. Also we are totally changing the streetfighter, we have a new design direction that I think you will appreciate with a totally unique dropout design that is really solid. (Stay tuned for that). I just finished up the solidworks drawings for it and we should hopefully get some prototypes early spring.

Jane McRee said...

Perfect and inspiring designing has never been possible in smaller time without availability of laser cutting technology. Metal in different measurements can be treated efficiently to give ideal shape and look to them.

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